keyboard: Show in an idle on clutter key focus changes

keyboard: Show in an idle on clutter key focus changes

It's common to do actor.grab_key_focus() before the actor is mapped
which means that we can't reliably determine where the actor is at
notify::key-focus time and thus might end up showing the keyboard on
the wrong monitor.

This is happening, in particular, with the run dialog. Delaying until
we hit the main loop allows us to know where the actor finally is
before showing the OSK.

https://bugzilla.gnome.org/show_bug.cgi?id=685856